Drop cable

ABSTRACT

PCT No. PCT/GB89/01438 Sec. 371 Date Jun. 25, 1991 Sec. 102(e) Date Jun. 25, 1991 PCT Filed Dec. 1, 1989 PCT Pub. No. WO90/07138 PCT Pub. Date Jun. 28, 1990A drop cable arrangement in which a carried line (9) is supported by a strength member (3), the carried line being attached to the strength member via a tubular member (7) that is a sufficiently loose fit around the strength member to permit relative longitudinal movement therebetween. In the event of additional loading on the drop cable the strength member extends due to the increased tension, but this tension is not transferred to the carried line due to the permitted relative movement of the tubular member and strength member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to suspended cables and the like, and inparticular but not exclusively to suspended transmission lines.

2. Description of Related Art

For above ground routes, a cable, known in this context as a drop cable,may be suspended from poles so that the cable hangs between eachadjacent pair of poles in a catenary. Typically, a drop cable comprisesa strength member made of tensile steel which supports the load, and acarried member, which may for example be a more delicate transmissionline or lines. The carried member is attached, either continually or atintervals, to the strength member. For transmission lines, the typicaldistance between poles is 200 feet, that is 69 metres. Between thepoles, the drop cable sags due to its own weight, the extent of the sagon installation being determined by the tension in the drop cable, andbeing designed to be within a range of values determined by theacceptable drop cable tension and the acceptable extent of eventual sagto avoid hazard. In addition to the suspension load of the weight of thedrop cable itself, an externally-mounted drop cable is subject toadditional variable loading due to wind force and settling of moistureor ice formation. This additional loading results in strain in the dropcable which will affect all the elements of the cable including thecarried line(s).

Optical fibre or other lightweight transmission lines may beconveniently installed in a previously-suspended drop cable the strengthmember of which supports a duct along which a lightweight package can beinstalled by the technique known as fibre blowing, this technique beingdescribed in our European Patent specification No. 108590.

This technique involves blowing compressed fluid, usually gas, along aduct into which the transmission line is to be installed, and feedingthe transmission line into the duct at the same time, so that it isurged along by the viscous drag of the fluid flow. A particular featureof this technique is that it enables sensitive transmission lines,especially those containing optical fibres, to be installed after thelaying, or suspending, of the duct, and the transmission line is,therefore, free from any stress resulting from installation orsuspension of the duct itself. Alternatively, the carried line may belashed to a previously-suspended strength member, but this is lessconvenient. However, even if a transmission line is installed aftersuspension of the strength member, it is in present systems stillsubjected to the additional, variable strains resulting from ice and/orwind loading on the drop cable.

SUMMARY OF THE INVENTION

An aim of the invention is to provide a convenient means for suspendinga carried member from a strength member in a catenary and to inhibitenvironmentally-produced strain on sensitive elements. Another aim is toprovide a means for reducing the strain experienced by sensitiveelements in a catenary system.

The present invention provides a drop cable arrangement for a catenary,the arrangement comprising a tensile, load-supporting, strength member,a carried member, and a tubular member supporting the carried member,the strength member being mounted within the tubular member, and thetubular member being of a sufficient internal size to enable relativelongitudinal movement between the strength member and the tubularmember.

The invention also provides a drop cable arrangement for a catenary, thearrangement comprising a tensile, load-supporting, strength number and acarried member, wherein an elastic linkage is provided in the strengthmember, the elastic linkage having an extension rate per unit lengthsubstantially greater than the extension rate of the strength member,and wherein the carried member is provided with sufficient slack to takeup the extension of the elastic linkage without strain.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described, by way of example, with referenceto the accompanying drawings in which:

FIG. 1 schematically illustrates a first embodiment of the invention;

FIG. 2 schematically illustrates part of the first embodiment;

FIGS 2a and 2b depict alternate exemplary embodiments;

FIG. 3 schematically illustrates a second embodiment of the invention;

FIG. 4 schematically illustrates a third embodiment of the invention;and

FIG. 5 is a cross-section through a simple drop cable for use with thethird embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a catenary system comprises poles 1 which each havea connection ring 2 to which a tensile load supporting strength member 3of a drop cable 4 is attached via a connection stopper member 5.Alternative means for attaching the strength member 3 to the poles 1 maybe utilised, the arrangement shown being typical of presently usedtransmission line attachments. The strength member 3 supports a carriedmember 6. A tubular member 7, which may be made integrally with thecarried member 6 or attached to it by other means, extends around thestrength member (more clearly seen by reference to FIGS. 2 and 3) and issufficiently loose to enable relative longitudinal movement between thetubular member and the strength member. Prior to attachment of thestrength member 3 to a pole 1, the carried member 6 separates from thestrength member and is left slack as it bypasses the pole, and thenrejoins the strength member of the next span after its attachment to thepole. In an external suspended drop cable system there are initialloading and tension factors which are determined by the drop cableweight and the selected installation tension, and superimposed on thisthere are changes in loading due to factors such as ice and wind.Typical values may be a total drop cable weight of 0.3 to 0.5 Newtonsper metre run, but environmental loading (caused for example by acladding of ice to a thickness of 5 mm and the ice clad cable beingsubjected to wind loading) may increase the weight respectively by up to2 to 3 and 5 to 6 Newtons per metre run. Thus, the environmental loadingmay greatly exceed the cable weight loading, and the increased loadimposes increased tension and consequent elongation in the suspendedcable. The total loading on a cable is given by: ##EQU1## and for thevalues mentioned above strains of the order of 0.3% or greater areexperienced, the precise value depending on the cross-sectional size andthe Youngs Modulus of the strength member 3. Strains of this extentcannot be tolerated in delicate transmission lines such as thosecontaining optical fibres.

With the arrangement shown in FIG. 1, it can be assumed that the load ofthe complete drop cable 4 is taken by the strength member 3. When theload increases, the strain is solely experienced by the strength member3 because the tubular member 7 slides upon the strength member by virtueof its loose fit and the tubular member's resistance to relativemovement with respect to the strength member being less than its ownresistance to extension. The slack in the carried member 6 is sufficientto take up the extension of the strength member 3 without strain. Thus,the tubular member 7, and any line carried within it, remainsubstantially free from environmentally induced strain, thereby enablingelements that are sensitive to strain such as optical fibres or otherdelicate transmission lines, to be carried in a drop wire system.

Referring now in more detail to FIG. 2, the drop cable 4 comprises twoside-by-side passageways constituting the carried member 6 and tubularmember 7. As shown, the two passageways are of equal size, but it ispossible for them to differ in size or for more passageways to beprovided, as shown for example in FIGS. 2a and 2b. Conveniently, thepassageways may be formed by an integral plastics extrusion ofside-by-side tubes. The strength member 3 passes along the passagewayconstituting the tubular member 7, this being normally achieved bythreading the strength member through the passageway, although formingThe passageway around the strength member may also be possible. Thepassageway constituting the carried member 6 supports a transmissionline 9 which, when installed, also forms part of the carried line. Theline 9 may be installed in this passageway prior to, or after,installation of the catenary. In the case of lightweight transmissionlines, and especially those containing optical fibres, it is convenientto install the line 9 in its passageway by the fibre blowing techniquedescribed in EP108590, and this technique is conveniently utilised aftersuspension of the drop cable 4 in the catenary.

To enhance blowability when using a fibre blowing technique, and forlong term retention of good blowing characteristics, it is convenient touse a lubricant-impregnated plastic for the drop cable 4. It may not bepossible to obtain good blowability and the mechanical propertiesdesired for the drop cable 4 from a single plastics material or from ablend of plastics materials. Optimum, or more nearly optimum properties,can be achieved if the passageway constituting the carried member 6 islined with a different plastics material from that used for the rest ofthe drop cable structure. The lining and the drop cable structure may beformed simultaneously, or almost simultaneously, using a co-extrusion orseries extrusion process respectively. Alternatively, the lining of thepassageway constituting the carried member 6 may be formed in a firstoperation, the remainder of the drop cable body being formedsubsequently (minutes, hours or days later as appropriate) by extrusionabout the lining. High density polyethylene (HDPE) is a preferred liningmaterial. Particularly preferred is HDPE incorporating a solid lubricantsuch as antistatic grade carbon. A concentration in the range of 5% to10% antistatic grade carbon is preferred, most preferably 8%. Typicalco-extrusion rates are of the order of 10 metres per minute. Where thelining is co-extruded, it will typically be 0.2 to 0.5 mm in thickness,more typically 0.25 to 0.35 mm. Where the lining is produced other thanin a co-extrusion process, the lining wall thickness may preferably besomewhat greater, for example up to 1 mm.

Typical internal diameters for the passageway constituting the carriedmember 6 are in the range 3 to 7 mm, preferably 5 to 6 mm. Thesedimensions are particularly suitable for use of fibre blowing processesfor installation of transmission lines such as suitably packagedmultimode or monomode optical fibres.

In FIG. 3, a modified tubular member 7 is illustrated which has anopenable side to enable the tubular member to be engaged around astrength member 3 where threading is not convenient. This hasapplicability to retrofitting around a previously suspended line (whichmay be a strength member only or a drop cable including a strengthmember) where access to the ends is not available. In the embodimentshown, the side of the tubular member 7 is openable by virtue of aparting line at the base of the right hand side (as viewed) and thenatural resilience of the material of the tube effectively forming ahinge at the top of the tubular member. The confronting sides of theparting line are provided with a cooperating ball and cup catch 10.

As previously mentioned, it is necessary for the resistance of thetubular member 7 to extension to be less than the sliding resistancewhich enables the strength member 3 to expand without stretching thetubular member. To this end, it is desirable to incorporate means toreduce the possibility of the strength member 3 becoming caught, orotherwise stuck in the tubular member 7. In general, the strength member3 will be made of a corrosion-resistant material such as stainlesssteel, which may be further coated or oiled (for example silicone oiled)to ease friction between the strength member and the tubular member 7,and/or further aid corrosion resistance. Low friction coatings, such assilicone or polytetrafluoroethylene may also be utilised for the insidesurface of the tubular member 7, which in general will comprise aplastics material. A potential source of sticking is icing of thestrength member 3 within the tubular member 7. To reduce the possibilityof this happening, vents 11 (FIG. 1) may be provided at the low point ofeach catenary span, the vents enabling egress of water from the tubularmember 7. In the vicinity of the vents 11, drip beads may be provided toaid channelling of drips away from contact with the strength member 3and the tubular member 7. Anti-wetting agents and/or low frictioncoating inside the tubular member aid(s) the egress of water. Drip beadsor other formations may also be used to discourage entry of water,either directly or by running along the strength member 3, into thetubular member 7 at its open ends. Additional strain resistance may beprovided by including at least one auxiliary strength member 12 (seeFIG. 3) in the wall of the tubular member 7. Preferably, the auxiliarystrength member 12 is made of polyaramide, is extruded into a plasticstubular member, and also provides resistance to temperature-inducedstrain by virtue of its negative coefficient of thermal expansion. Anauxiliary strength member may also be provided in the embodiment shownin FIG. 2. The auxiliary strength member 12 may continue across theslack of the carried member 6.

It will be realised that the degree of slack in the carried member 6between its attachment points to adjacent spans of the strength member 3needs to be sufficient to take up any elongation in the strength memberdue to environmental loading and/or any sliding of the tubular member 7to an asymmetric position on the span. For convenience of installation,the carried member 6 may be provided in lengths, and the lengthsconnected by coupling with tube connectors 13 (FIG. 1). Where a "blownfibre" installation technique is used to install the transmission linein the carried member 6, it is important for the tube connectors 13 toform well-sealed joints to the members, so that leakage of the gas usedfor blowing is avoided. Of course, for short lengths, some leakage cangenerally be tolerated.

If the passageways constituting the tubular member 7 and the carriedmember 6 are formed integrally, such as by a plastics extrusion,separation, termination and relative shortening of the lengths oftubular member may be performed at the point of installation.

The tubular member 7 may be fixed relative to the strength member 3 atone end, or at some intermediate location, in order to control thedirection of sliding, or to prevent unrestricted relative longitudinalsliding.

FIG. 4 shows a third embodiment of the invention in which each of thestrength members', 31 is attached to one of the poles supporting thedrop cable system via an elastic linkage, provided in this example by ahelical spring 33. Conveniently, the spring 33 is attached to the polevia a ring mounting 34 to which an elastic linkage (spring) 33 of thenext span or catenary length is also attached. Preferably, each catenarylength of strength member 31 is provided with an elastic linkage 33 ateach end. With this arrangement, when the drop cable is subjected toadditional loading, the elastic linkages 33 extend and, as will be shownlater, thereby enables reduction in the strain experienced in the cable.The carried member 32 for the transmission line (not shown) must becontinuous, and so, where an elastic linkage 33 is attached to theadjacent strength member 31 (at point 53) the carried member isseparated from the strength member (now constituted by the elasticlinkage), and continues separately from the elastic linkage for a shortdistance until it rejoins the strength member after the elastic section.In the embodiment shown, the elastic linkage 33 at the end of the nextadjacent catenary length is also bypassed in a continuous loop beforethe carried member 32 rejoins the strength member 31. An auxiliarystrength member (not shown but similar to the member 12) may be providedin the carried member 32 to aid in its support between the twoattachment points 53 of the elastic linkages 33 to the strength member31. The loop of unsupported carried member 32 is provided with a degreeof slack that is at least equal to the maximum extension that theelastic linkages 33 that it bridges will undergo.

The invention has been illustrated by the provision of elastic linkages33 between the strength members 31 and the poles. This enables provisionof two elastic linkages 33 for each catenary length, with minimuminconvenience, since the strength members 31 have, in any event, to beinterrupted for secure connection to the poles. However, a more generalprincipal of the invention is to provide an elastic linkage anywhere inthe strength member, and for the carried member to be separated from thestrength member for at least the length of the elastic linkage andprovided with sufficient slack to accommodate the maximum expansion ofthe elastic linkage. Such an elastic linkage or linkages could beprovided at any location in each catenary length.

In any external suspended drop cable system, there are initial loadingand tension factors which are determined by the drop cable weight andthe selected installation tension, and superimposed on this there arechanges in loading due to factors such as ice and wind. It isestablished that the relationship in a catenary between the catenarydrop (the maximum sag) D, the distance L between poles, the distributedload W per metre run on the drop cable, and the tension T in the dropcable can be expressed as:

    D=L.sup.2 W/8T                                             (1)

from which it can be seen that the drop increases with pole separationand load, and decreases with tension. The principal of the presentinvention is to eliminate, or reduce, changes in the tension T byallowing an increase in the catenary drop D, in order to compensate forthe additional loading which would affect W in the above equation.

While it is preferred to use the previously described drop cabletogether with the sprung catenary system, it is possible to use thesprung catenary system with a `solid` drop cable of the type shown inFIG. 5. In such a drop cable, there is no freedom for relative movementbetween the strength member 3 and the surrounding plastics material. Asbefore, the carried member 32 is intended for the installation oftransmission lines by means of a `blown fibre` technique, subsequent toinstallation of the drop cable. Consequently, the details given abovewith reference to the carried member 32 of the drop cables of FIGS. 1, 2and 3, apply equally to the drop cable of FIG. 5.

The effectiveness of a resilient link in the strength member is nowdemonstrated numerically using the simple drop cable shown in FIG. 5 inthe arrangement shown in FIG. 4. In the event that the drop cable ischanged to a different design, or the installation tension or spanlength is changed, then the numerical values will change, and it may benecessary to make corresponding changes in the available extension inthe elastic linkages.

For the purposes of calculating the maximum load on the cable, it isassumed that there is a 5 mm coating of ice over the surface of thecable, and that this ice-clad cable is subjected to a wind loading of 80km/hr. It is further assumed that the entire load is borne by thestrength member, and that the relationship of Young'sModulus=stress/strain holds for the tensile steel strength member.

In the drop cable of FIG. 5, the steel strength member has across-sectional area of 1.7671 mm². The total weight of the cable,including an installed fibre package, is 0.364 Newtons per metre run,and 5 mm of radial ice adds a weight of 1.98 Newtons per metre run togive a total ice clad weight of 2.344 Newtons per metre run. Using thefactors and formulae for wind loading from Constrado, Publication 1/75(1975), `Wind forces on unclad tubular structures`, the ice-clad cablepresents an effective size of 0.015 m, and an 80 km/hr wind loadprovides a load of 5.438 Newtons per metre run. ##EQU2##

For this purpose of the example, a maximum distributed load of thisvalue, 5.92 Newtons per metre run, is now assumed.

In order to ascertain the required properties of the elastic linkage, itis necessary to find the required extension to increase the catenarydrop sufficiently to keep the tension and hence the strain withinacceptable limits.

Upon installation, i.e. under cable load only (no wind or ice), acatenary drop of 0.7 metres is an acceptable standard. Using this valuein equation (1), with the cable weight of 0.364 Newtons per metre runfor a span length of 68 metres provides:

    0.7=68.sup.2 ×0.364/8T

which gives an installation tension T=300.56 Newtons. Using an iterativecomputer program, it can be demonstrated that, in order to support adistributed load of 5.92 Newtons assumed above, a tension of 1350Newtons is required. Thus, the ice and wind load provides an increase inload of 1050 Newtons.

In the absence of any springs, using the relationship YoungsModulus=stress/strain for the increase in tension in the strengthmember, a value of 160×10 Newtons per square mm for Young's Modulus forthe tensile steel, and a cross-sectional area of 1.7671 square mm forthe strength member gives:

    strain=load/(area×Young's Modulus).

    strain=1050/(1.7671×160×10.sup.3)=3.7137×10.sup.-3 .

Thus, the additional strain is 0.37% which is far in excess of thatpermissible for an optical fibre. Only the additional strain due to theice and wind loading has been considered, as it has been assumed thatthe optical fibre was installed after suspension of the drop cable, andis therefore not subject to the drop cable installation tension of 300Newtons.

If we take a maximum acceptable strain limit of 0.25% (which is in facttoo high for optical fibres but serves to illustrate the point), thenthe maximum catenary length that is acceptable can be calculated.

The catenary length of the unloaded, installed cable is calculatedfirst, since again it is only changes from that length that affect asubsequently-installed fibre.

    ______________________________________                                        Catenary length = L + (W.sup.2 L.sup.3)/24 T.sup.2                            installed catenary length =                                                   68 + (0.344.sup.2 × 68.sup.3)/(24 × 300.sup.2) = 68.0172          meters                                                                        A 0.25% elongation gives                                                      68.0172 × 1.0025 = 68.1873 meters                                       ______________________________________                                    

Thus, to support a 5.92 Newton per metre run distributed load with onlya 0.25% strain increase, the following is known:

    ______________________________________                                        initial installation tension = 300 Newtons                                    distance between poles = 68 meters                                            distributed weight (under = 5.92 Newtons per meter run                        maximum ice/wind loading)                                                     permissible catenary length = 68.1873 meters                                  (calculated above)                                                            ______________________________________                                    

The additional tension in a cable extended to its maximum permissiblestrain (0.25%) can be found using the relationship YoungsModulus=stress/strain for the increase in strain, and substitutingTension=stress×area gives the relationship.

Tension=Youngs Modulus×strain×area and, using the values given earlier,this gives Tension=160×10³ ×0.0025×1.7671=706.84 Newtons

In other words, a 0.25% strain limit only enables a maximum tension inthe cable of 1006.84 Newtons (i.e. the installation tension of 300Newtons plus the increase of 706.84 Newtons calculated above).

In order to reduce the tension from the previously calculated maximumtension of 1350 Newtons to the permissible tension of 1006.84 Newtons(so as not to exceed the 0.25% strain) the catenary length has toincrease. This increase in length cannot be provided by the cable itself(as this would increase the strain), but is provided by the springs.

The catenary length required to reduce the tension sufficiently can beobtained from the relationship: ##EQU3## where L=distance between poles

W=weight per metre run

T=tension

and, using the established values, this gives catenary ##EQU4## Thus,the required catenary length, in order to yield a maximum tension of1006.84 Newtons for a 5.92 Newtons per metre run load, is 68.4529metres.

The maximum length of the cable for 0.25% strain (which will support1006.84 Newtons) is 68.1873 metres.

Therefore, the springs (or other elastic linkages) must provide anextension of the additional catenary length required,68.4529-68.1873=0.2656 metres over a tension charge of 706.84 Newtons.If springs are provided at each end of each span, this yields a requiredrate of 5.31 N/mm for each spring.

It is, of course, possible to provide springs that would take up theinstallation strain in the event that the fibre was not subsequentlyinstalled, or for a different maximum strain. If the maximum strain isabout half that allowed above, i.e. 0.125% for example, then the springextension rate would need to be approximately doubled.

The embodiment described utilises a mechanical spring, but theelasticity may be provided by other means such as an elastic polymermaterial i.e. in the form of an entropy spring rather than an energyspring.

In FIG. 4 it will be observed that each spring 33 is connected to itsattachment point 53 via a stopper member 36. Each stopper member 36comprises a small diameter (for example 1 or 2 cm) helix with acomparatively long pitch length, so that the turns are extended andopen.

The end of the drop cable strength member 31 from the drop cable iswound around the open turns of the helical stopper member 36 so that,under tension, the strength member pulls tightly against the turns ofthe stopper member and is secured therein. This type of securement of astrength member in a helical stopper member is well known in existingcatenary systems. In order to aid gripping, the surface of the stoppermember 36 may be coated with a high friction material such ascarborundum powder or a PVC moulding. A progressive or distributedgripping action may be provided by having a varying diameter helix, withthe turns tapering from a larger diameter at the end where the strengthmember 31 is introduced. The helical stopper member 36, in order tofunction to grip the drop cable strength member 31, must becomparatively rigid with respect to the strength member. In aparticularly preferred embodiment of the invention, the stopper member36 is formed integrally with the associated elastic linkage 33 (althoughit is possible to use a stopper member which is separate from, butconnected to, an elastic linkage). It is possible for the same diameterof wire (for example standard 10 gauge) to form the rigid stopper member36 by virtue of small diameter turns, and then be formed intosubstantially larger turns to provide the elastic linkage. It will berealised that, within the general context of the expression 'strengthmember', the stopper member and the elastic linkage will comprise, orconstitute, the strength member by virtue of their attachment to, orcontinuation from, the drop cable strength member.

A further advantage of incorporating an elastic linkage, and one ofgeneral applicability, is that, upon initial installation, the springshould extend to an initial length corresponding to the desiredinstallation tension (assuming installation is carried out withoutsignificant environmental loading), and is effectively a built-intension gauge. This is particularly relevant where elastic linkages areused to reduce loading, since the calculation of the required springrate assumes a given installation tension, and, if the installationtension differs, then the correct degree of tension relief may not beprovided. However, the use of an elastic linkage purely as a tensiongauge for use in installing other catenaries where tension relief is notnecessary may also be useful.

We claim:
 1. A drop cable arrangement for a catenary, the arrangementcomprising:a tensile-load-supporting strength member and a carriedmember; an elastic linkage being provided in the strength member, theelastic linkage having an extension rate per unit length substantiallygreater than the extension rate of the strength member; the carriedmember being decoupled from the strength member along substantially allof the carried member whereby limited but significant elongation of saidstrength member can occur without significant elongation of the carriedmember, the carried member being provided with sufficient slack to takeup the extension of the elastic linkage without strain; in which theelastic linkage is a spring provided at the end of a catenary length,and forming part of an attachment means for attaching the strengthmember to a supporting pole, the attachment means further comprising ahelical stopper member, the spring being disposed between the helicalstopper member and a supporting pole; and in which the spring is ahelical spring formed integrally with the stopper member.
 2. A dropcable arrangement for a catenary, the arrangement comprising:atensile-load-supporting strength member and a carried member; an elasticlinkage being provided in the strength member, the elastic linkagehaving an extension rate per unit length substantially greater than theextension rate of the strength member; the carried member beingdecoupled from the strength member along substantially all of thecarried member whereby limited but significant elongation of saidstrength member can occur without significant elongation of the carriedmember, the carried member being provided with sufficient slack to takeup the extension of the elastic linkage without strain; and in which theor each elastic linkage provides an extension of at least 2.5 mm perNewton, over a catenary span.
 3. A drop cable arrangement for acatenary, the arrangement comprising:a tensile-load-supporting strengthmember and a carried member; an elastic linkage being provided in thestrength member, the elastic linkage having an extension rate per unitlength substantially greater than the extension rate of the strengthmember; the carried member being decoupled from the strength memberalong substantially all of the carried members whereby limited butsignificant elongation of said strength member can occur withoutsignificant elongation of the carried member, the carried member beingprovided with sufficient slack to take up the extension of the elasticlinkage without strain; a tubular member supporting the carried member;and in which the tubular member is provided with at least one vent orinterruption to permit egress of water.
 4. A drop cable arrangement fora catenary, the arrangement comprising:a tensile-load-supportingstrength member and a carried member; an elastic linkage being providedin the strength member, the elastic linkage having an extension rate perunit length substantially greater than the extension rate of thestrength member; the carried member being decoupled from the strengthmember along substantially all of the carried member whereby limited butsignificant elongation of said strength member can occur withoutsignificant elongation of the carried member, the carried member beingprovided with sufficient slack to take up the extension of the elasticlinkage without strain; a tubular member supporting the carried member;and in which the tubular member is provided with an auxiliary strengthmember; and in which the auxiliary strength member is made ofpolyaramide.
 5. A suspended cable arrangement comprising:first andsecond supports, a continuous, elongate, tensile-load-carrying strengthmember extending between and secured to said supports, the strengthmember supporting a duct, suitable for use in a fibre blowing process,the strength member not being received within said duct, and said ductbetween said supports being decoupled from the strength member alongsubstantially all of the strength member, whereby limited butsignificant elongation of said strength member can occur withoutsignificant elongation of said duct.
 6. A cable arrangement as in claim5 including three or more supports and wherein:the duct provides acontinuous path between all the supports, the duct diverges from thestrength member at the support, an excess length of the duct beingprovided at the support, wherein the duct comprises plural discreteparts joined together in an end-to-end relationship, the joints betweenthe parts being substantially gas tight.
 7. A cable arrangement as inclaim 5 wherein a tubular member loosely surrounds the strength memberin a first cavity, the duct being formed as a second cavity distinctfrom the first, and the duct being supported by said strength member bymeans of said tubular member.
 8. A cable arrangement as in claim 7wherein the tubular member and the duct are each part of a one piecebody of plastics material, the body of the duct being provided byplastics materials which is different that which provides the innersurface of the duct.
 9. A cable arrangement as in claim 8 wherein theinner surface of the duct is provided by plastics material incorporatinga lubricant.
 10. A cable arrangement as in claim 9 wherein the lubricantis antistatic grade carbon.
 11. A suspended cable arrangementcomprising:first and second supports, a continuous, elongate,tensile-load-carrying strength member extending between and secured tosaid supports, the strength member supporting a duct, suitable for usein a fibre blowing process and having a bore diameter in the range of 3to 7 mm, the strength member not being received within said duct, andsaid duct between said supports being decoupled from the strength memberalong substantially all of said strength member, whereby limited butsignificant elongation of said strength member can occur withoutsignificant elongation of said duct.
 12. A suspended cable arrangementcomprising:first and second supports, a continuous, elongatetensile-load-carrying strength member extending between and secured tosaid supports, the strength member supporting a duct, suitable for usein a fibre blowing process and having a bore diameter in the range of 3to 7 mm, the strength member not being received within said duct, saidduct being decoupled from the strength member along substantially all ofsaid strength member, whereby limited elongation of said strength membercan occur without significant elongation of said duct; a tubular memberloosely surrounding said strength member, the duct being supported bysaid strength member by means of said tubular member, and the tubularmember and the duct each being part of a one-piece body of plasticsmaterial, the body of the duct being provided by a plastics materialwhich is different from that which provides the inner surface of theduct.
 13. A drop cable capable of being suspended from poles in acatenary, comprising:a duct suitable for use in a fibre blowing process,means connected to or integral with said duct to engage with acontinuous , elongate tensile-load-carrying strength member, said ductbeing decoupled from the strength member along substantially all of saidstrength member whereby limited elongation of said strength member canoccur without significant elongation of said duct.
 14. A drop cable asclaimed in claim 13, wherein said means comprises a tubular member intowhich the strength member can be received.
 15. A drop cable arrangementfor blown installation of optical fibre thereinto, said arrangementcomprising:an elongated strength member which carries an elongatedoptical fibre duct in plural s pans disposed between successive elevatedpoints, said fibre duct being suitable for blown fibre installationthroughout the plural spans of an installed drop cable arrangement; saidstrength member being mechanically decoupled from said fibre duct in theelongated longitudinal dimension along substantially all of saidstrength member so that changes in length of the strength member causedby changes in temperature and loading stress produce substantially less,if any, strain in the fibre duct; and said duct being routed around saidelevated points with sufficient slack to accommodate changes in strengthmember length while remaining substantially gas-tight at any connectionjoint thereof between duct spans so as to remain suitable for blownfibre installation throughout the plural spans.
 16. A method ofinstalling optical fibre along a drop cable arrangement spanning pluralvertical support points spaced apart over an extended path, said methodcomprising the steps of:installing a drop cable along said extended pathbetween said vertical support points, said drop cable having anelongated strength member carried by said vertical support points and,in turn, carrying an elongated optical fibre duct which is mechanicallydecoupled from the strength member in the longitudinal direction alongsaid path along substantially all of said strength member so thatchanges in the length of the strength member caused by changes intemperature and loading stress produce substantially less, if any,strain in the fibre duct; effecting gas-tight joints, if needed, in theduct which bypass the vertical support points, and thereafter installingan optical fibre transmission line into said duct along said path usingblown fibre techniques that urge the fibre from one end of the ductusing viscous friction between the fibre and faster moving gases alongsaid duct.
 17. A suspended cable arrangement comprising-first and secondsupports,a continuous, elongate, tensile-load-carrying strength memberextending between and secured to the supports, the strength membersupporting a duct suitable for use in a fibre blowing process, thestrength member not being received within the duct, the duct between thesupports being decoupled from the strength member along substantiallyall of its length, whereby limited but significant elongation of thestrength member can occur without significant elongation of the duct,and wherein the duct comprises plastics materials, a body of the ductcomprising a first plastics material, and an inner surface of the ductbeing provided by a second plastics material which is a polymerincorporating a solid lubricant.
 18. A cable arrangement as claimed inclaim 17 wherein there are three or more supports, the duct providing acontinuous path between all the supports, the duct diverging from thestrength member at the supports, and excess length of the duct beingprovided at the supports, wherein the duct comprises plural discreteparts joined together in an end-to-end relationship, the joints betweenthe parts being substantially gas tight.
 19. A cable arrangement asclaimed in claim 17 wherein a tubular member loosely surrounds thestrength member in a first cavity, the duct being formed as a secondcavity distinct from the first, and the duct being supported by thestrength member by means of the tubular member.
 20. A cable arrangementas claimed in claim 19 wherein the tubular member and the duct are eachpart of a one piece body of plastics material, the body of the ductbeing provided by plastics material which is different from that whichprovides the inner surface of the duct.
 21. A cable arrangement asclaimed in claim 18 wherein the second plastics material is high densitypolyethylene.
 22. A cable arrangement as claimed in claim 17 wherein thelubricant is an antistatic grade of carbon.
 23. A drop cable arrangementcomprising a duct suitable for use in a fibre blowing process, and meansconnected to or integral with the duct for engagement with atensile-load-carrying strength member, said tensile-load-carryingstrength member being decoupled from the duct along substantially all ofits length wherein the duct comprises plastics materials, a body of theduct comprises a first plastics material and an inner surface of theduct is provided by a second plastics material, the second plasticsmaterial being a polymer incorporating a solid lubricant.
 24. A dropcable arrangement as claimed in claim 23 wherein the second plasticsmaterial is high density polyethylene.
 25. A cable arrangement asclaimed in claim 23 wherein the solid lubricant is an anti-static gradeof carbon.
 26. A cable arrangement as claimed in claim 25, wehrein theconcentration of carbon in the second plastics material is in the range5 to 10 per cent.
 27. A cable arrangement as claimed in claim 23 whereinthe inner surface is provided by a lining having a thickness in therange 0.2 to 1 mm.
 28. A drop cable arrangement for blown installationof optical fibre thereinto, the arrangement comprising:an elongatestrength member which carries an elongate optical fibre duct in pluralspans disposed between successive elevated points, the fibre duct beingsuitable for in situ blown fibre installation throughout the pluralspans of an installed drop cable arrangement; said duct comprisingplastics materials, a body of the duct comprising a first plasticsmaterial, and an inner surface of the duct being provided by a secondplastics material which incorporates a solid lubricant; said strengthmember being mechanically decoupled from the fibre duct in the elongatedlongitudinal dimension along substantially all of its length so thatchanges in length of the strength member caused by changes intemperature and loading stress produce substantially less, if any,strain in the fibre duct; and said duct being routed around the elevatedpoints with sufficient slack to accommodate changes in strength memberlength while remaining substantially gas-tight at any connection jointthereof between duct spans so as to remain suitable for blown fibreinstallation throughout the plural spans.
 29. A method of installingoptical fibre along a drop cable arrangement spanning plural verticalsupport points spaced apart over an extended path, the method comprisingthe steps of:installing a drop cable along the extended path between thevertical support points, the drop cable having an elongated strengthmember carried by the vertical support points and, in turn, carrying anelongated duct which is mechanically decoupled from the strength memberin the longitudinal direction along substantially the entire length ofthe path so that changes in the length of the strength member caused bychanges in temperature and loading stress produce substantially less, ifany, strain in the fibre duct; said duct comprising plastics materials,a body of the duct comprising a first plastics material, and an innersurface of the duct being provided by a second plastics material whichincorporates a solid lubricant; effecting gas-tight joints, if needed,in the duct which bypass the vertical support points; and thereafterinstalling an optical fibre transmission line into the duct along thepath using blown fibre techniques such that the fibre is propelled fromone end of the duct using viscous friction between the fibre and fastermoving gases along the duct.